Vanishing of text while copying a scroll

[Dryder]

Why do the runes disappear from a scroll if a caster copies a spell from it?
I just want to understand the reasoning behind that rule?
If I copy a spell from another mages spellbook, the writing does not disappear.

[Chemlak]

Because a scroll is a one-use version of a spell. In the same way that writing a prepared spell into a spell book wipes it from the caster's mind, transferring it from a scroll (which is just like a prepared spell slot in written form) wipes it from the scroll.

[CBDunkerson]

They are two different things. Note that you also can't cast the spell directly from the spell book as you could with a scroll. The letters on the scroll ARE the magic item (the scroll itself is just a piece of parchment). When that magic is expended (either to cast the spell or allow you to write a version of it into a spell book) the letters disappear... the magic is only usable once. Spells in a spell book on the other hand are NOT magic items. The letters are just ink on a page. No magic to be expended.

[RDM42]

And that is what vancian spell casting is in d&d - you are basically using your mind as a multi spell scroll, when 'memorizing' you are writing the spells to your brain and when you cast them they disappear. The difference in a scroll is you are 'writing' the spell to an external object.

[Dryder]

Thank you, guys.
That makes sense. Mind eased... :)